Structural, electrical and magnetic properties of Mg-Zr co-substituted Ni0.5Zn0.5Fe2O4

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Original Article

Structural, electrical and magnetic properties of Mg-Zr co-substituted

Ni

_0.5

Zn

_0.5

Fe

₂

O

₄

K. Jalaiah

a,b,*

, K. Chandra Mouli

c

, K. Vijaya Babu

d

, R.V. Krishnaiah

e
a_{Chebrolu Engineering College, Chebrolu, Guntur, 522212, India}

b_{Department of Physics, Andhra University, Visakhapatnam 530003, India}

c_{Department of Engineering, Physics, Andhra University, Visakhapatnam 530003, India}
d_{Advanced Analytical Laboratory, Andhra University, 530003, India}

e_{Institute of Aeronautical Engineering and Technology, Hyderabad, 500043, India}

a r t i c l e i n f o

Article history:
Received 2 October 2018
Received in revised form
15 December 2018
Accepted 16 December 2018
Available online 23 December 2018

Keywords:
Ferrites
XRD
TEM
SEM
Permeability

Saturation magnetization
Anisotropy constant

a b s t r a c t

Zr and Mg co-substituted Ni0.5Zn0.5Fe2O4ferrites have been synthesized by the sol-gel auto-combustion

method. The X-ray diffraction patterns evidenced the single phase cubic spinel structure. The lattice
parameter and cell volume are in resemblance trend with the variation of the dopant concentration. The
similar trend is observed for the crystallite and particle size. The porosity and sintered density, however,
vary in an opposite way with a variation of the dopant concentration. The same variation is found for the
drift mobility and DC resistivity. The Arrhenius graphs of DC resistivity exhibit the semiconductor nature,
for which the activation energy decreased with increasing the dopant concentration. Moreover, as the
dopant contents increased, the saturation magnetization, net magnetic moment and permeability are
reduced, while the coercivity is reinforced. Thesefindings can be correlated with the variation of the
porosity and grain size.

© 2018 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />

1. Introduction

In early days iron based magnetic alloys are used in various
applications. However, their low resistivity made these materials
inefficient at high frequencies, which encouraged the eddy current
through them. This wasted energy is created a serious problem that
generated the heat in the circuit. Hence, iron based magnetic
ma-terials are not favorable in high frequency applications. Ferrite
materials, in opposite, possess high resistivity and dielectric
per-formances and do not conduct the electric current readily. The
advantage of ferrites over magnetic alloys is that they formed a

different combination of ferrites with transition metals because the
transition metals exhibit magnetic as well as semiconductor
properties. The porosity is an insignificant factor for ferrites so that
the ferrites have been investigated for several years based on this
issue. In order to get the high resistivity of ferrites researchers

choose different combination here we also choose a new
combi-nation with transition metals to get the high resistivity of ferrite
material[1,2]. Spinal ferrites are a class of magnetic oxides with the
general formula of AB2O4. They are categorized as soft and hard
ferrites according to their magnetic performance. Soft ferrites are
easily demagnetized without signi_{ficant energy need, i.e. only a}
small energy amount is wasted in the form of eddy currents to
demagnetize the soft magnetic materials. In case of hard ferrites, a
significantly higher energy is needed to demagnetize. This means
that soft magnetic materials possess higher electrical resistivity,
thus, they are used in inductors and transformers. The magnetic
oxides are made from the blend of iron, nickel, zinc, manganese
oxides. By using these oxides, different combinations of soft ferrites
like Manganese-Zinc and Nickel-Zinc have been prepared. For
inductor cores, the magnetic permeability is the chief parameter

[3,4]. In order to improve the core performance at high frequency
the grain size, which can be controled by the ferrite preparation
technique, plays an important role. The solid state ceramic
tech-nique is a general ferrite fabricated techtech-nique, in which the
con-stituent oxides react at higher temperatures. In this case, an
unusual grain growth usually occurs due to the non stoichiometry

* Corresponding author. Chebrolu Engineering College, Chebrolu, Guntur,

522212, India.

E-mail address:(K. Jalaiah).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j s a m d

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and inhomogeneity of ferrite materials[5]. To control this unusual
grain growth, we adopted the solution method known as the
sol-gel autocombustion method in which the constituent oxides react
at lower temperatures. So, the precursor material becomes
stoi-chiometry and homogeneity with controlled grain size. In the
present study, the correlation between structural, electrical and
magnetic properties of Mg-Zr co-substituted Ni0.5Zn0.5Fe2O4 are
discussed in connection with the dopant concentration.

2. Experimental

The Zr and Mg co-substituted Ni0.5Zn0.5ZrxMgxFe2-2xO4have
been prepared by sol-gel auto combustion method, x values vary
from the 0.08 to 0.4 in steps of 0.08 with%. The starting
mate-rials of all metal nitrates with AR grade Nickel nitrate
(Ni(NO3)2.6H2O), zinc nitrate (Zn (NO3)2.6H2O), Magnesium
ni-trate (Mg(NO3)2.6H2O), Zirconyle nitrate (ZrO (NO3)2), ferric
citrate (Fe C6H8O7.H2O) and citric acid (C6H8O7.H2O) are used for

synthesis of Ni0.5Zn0.5ZrxMgxFe2-2xO4(x¼ 0.08, 0.16, 0.24, 0.32,
0.4). The stoichiometric weights of metal nitrates dissolved in
deionized water and the citric acid added to the solution as per
the oxygen ions present in chemical formula, later 50 ml
ethylene glycol added to the solution[6]. The ammonia solution
added drop wise to adjust the PH value of 7 for thefinal
solu-tion. Then the neutralizing solution heated to 600oCe700_{C for}

8e10 h with continuous stirring. After 8e10 h the solution

turned into a viscous on the formation of gel, then the
tem-perature of a gel rise to 100C drying,finally a powder form of

samples obtained [7]. The obtained powders processed for

simple experimental needs.
3. Structural studies

Fig. 1 shows the XRD patterns of Mg and Zr co-substituted
Ni0.5Zn0.5Fe2O4. Here the XRD patterns provide the evidence for
single phase cubic spinel and no extra peaks are observed
throughout while the doping concentration is increased. The lattice
constant is calculated from XRD peaks, using the following
equation.

a¼ dph2_{ỵ k}2_{ỵ l}2

where d is the space between the lattice planes. The lattice constant
and cell volumes are shown inFig. 2a with a variation of dopant
concentration. The increase in the lattice constant has resulted in

mismatches between the substitute ions and host ions ionic radius.
The Fe3ỵ (0.67) ions radius is small when compared with Zr
(0.80Å) and Mg (0.72 Å) ionic radii. Hence the substitution of Zr
and Mg in place of the Fe3ỵions unit cell will bulge promptly and as
a result the lattice constant increases with increasing dopant
con-centration[8]. The X-ray density is estimated from the following
equation.

Dx¼ 8M
Naa3

where M is the molar mass, Na is the Avogadro number and“a”
is the lattice constant. The X-ray density increases with increasing
dopant concentration from x¼ 0.08 to x ¼ 0.32, later it is slightly
decreased to x_{¼ 0.4. However, overall X-ray density increases with}
increasing dopant concentration. The increase in X-ray density may
be due to the lattice constant which is dominated by the molar
mass as shown in the above equation because the increase in the
lattice constant decreases the X-ray density. The ratio between the
sintered density and X-ray density gives the porosity of prepared
samples. The porosity of prepared samples is estimated from the
following equation.

p¼ 1 ds
dx

where_“ds” and “dx” are sintered and x-ray densities respectively.
The porosity and sintered density are shown in Fig. 2b with a
variation of dopant concentration. FromFig. 2b it is clear that both

the parameters exhibit opposite trend with a variation of dopant
concentration. The sintered density is decreased as a result of
lag-ging the sintering rate of material. The sintering rate of material is
lagging due to the volatilization of zinc at higher temperature. Since
the melting point of zinc is less than those of other constituent ions,
the material becomes non stoichiometry[9]. To minimize the non
stoichiometry property of the material, the excess of ferric oxide is
changed as ferrous oxide, i.e., Fe3ỵions are changed as Fe2ỵions in
the sintering process. The presence of Fe2ỵions in the material lags
the sintering rate of material, hence sintered density is decreased

[10]. The decrease in the sintered density results in the
develop-ment of pores in the material.

4. Surface morphology

The crystallite size is estimated from the following equation.

D¼0:94*

_b

l

cos

q

where

l

is the wavelength of Cu radiation and

b

is the full width
half maximum of (3 1 1) peak. The FWMH is decreased with
increasing substituting ionic radii so that the decrease in the
FWHM increases the crystallite size since crystallite size and
FWHM are inversely related in the above equation.Fig. 2c shows
the crystallite size and particle size with a variation of dopant
concentration. The TEM pictures are shown inFig. 3. The particle
size is measured from TEM pictures by using image-j software.
The particle size increased with increasing dopant concentration

as a result of the agglomeration nature of crystallites[11]. From

Fig. 2c we conclude that both the crystallite size and the particle
size are in comparable nano size. The SEM micrographs of
pre-pared samples are shown inFig. 4. The grain size is measured from
SEM micrographs by using the image-j software. The grain size
decreased with increasing dopant concentration. The decrease in
grain size is due to the development of pores in the material
during the sintering of material.

Fig. 1. X-ray diffraction patterns of Ni0.5Zn0.5ZrxMgxFe2-2xO4samples with x¼ 0.08,

0.16, 0.24, 0.32, and 0.4.

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5. DC resistivity

The ferrites exhibit the semiconducting nature since ferrites are
composed of transition elements and all transition elements show
the semiconducting nature.Fig. 5shows composition variation of
DC resistivity and drift mobility of prepared ferrite samples. The DC
resistivity of ferrite samples decreased with increasing doping
concentration. The decrease in DC resistivity because of the
increased electronic conduction between the paramagnetic region
(Fe2ỵions) to ferromagnetic (Fe3ỵions) region [12]. That is the
electronic exchange has occurred in ferrites from Fe2ỵ(n-type) to
Fe3ỵ(p-type). InFig. 5the drift mobility varies opposite to DC
re-sistivity with increasing doping concentration, since the decrease
in DC resistivity increases the mobility of electrons. The Arrhenius
plots drawn between DC resistivity and inverse temperature in the
range of 300 K and 620 K show that all plots are less curved. So

these plots reveal the semiconducting nature of prepared samples.
The Arrhenius plots for the present study are shown inFig. 6. The
semi conductivity of ferrite samples is described by the following
equation.

s

¼

s

oexp


_D

E

k

BT



where

s

o is the constant,

D

E is the activation energy, KB is the
Boltzmann constant and T is the absolute temperature. The graph

between

s

and 1/T gives more or less a curved line. The

D

E equals
0.1eV for stoichiometry composition and

D

E reaches 0.5eV for low
conductivity ferrites[13]. For the present study the Activation
en-ergy

D

E decreased from 0.17eV to 0.11eV i.e. the activation energy,
decreased with increasing dopant concentration and it is shown in

Fig. 7. The decrease in activation energy is due to increase of
jumping frequency of electrons from the paramagnetic region
(Fe2ỵ) to the ferromagnetic region (Fe3ỵ)[14].

6. Magnetic properties

The magnetic properties of Mg and Zr substituted Ni0.5Zn0.5

-Fe2O4ferrites are calculated by using the M-H loops shown in

Fig. 8. All the M-H loops are with less loss of magnetic energy and
the M-H loop data is collected at room temperature. The
satura-tion magnetizasatura-tion and the corresponding net magnetic moment
are estimated from M-H loops. Both the saturation magnetization
and the corresponding net magnetic moment are in decreasing
trend with increasing dopant concentration as shown inFig. 9a.
The decrease in saturation magnetization is due to the decrease of
Fe3ỵions in general formula with the substitution of Mg and Zr
ions in place of Fe3ỵions. The presence of Fe3ỵions in the material
needs a much moreux to orient in the applied eld direction.
Since Fe3ỵions behave like ferromagnetic ions the Fe3ỵions need

higher ux density to orient in the eld direction [15]. The

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decreased Fe3ỵions in material need lesserflux density to orient in

the field direction. Hence the saturation magnetization is

decreased with increasing dopant concentration. Father the net
magnetic moment of the material is calculated using the following
equation.

M¼ MA MB

here MAand MB are the magnetic moments of A-site and B-site
respectively. From the above equation the resultant magnetic

Fig. 3. Transmission electron micrographs of Ni0.5Zn0.5ZrxMgxFe2-2xO4along with selected area Electron diffraction patterned of samples.

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Fig. 4. Schematic SEM photo graphs of Zr and Mg Co substituted Ni0.5Zn0.5ZrxMgxFe2-2xO4.

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moment of material is the difference between the B-site magnetic
moment and A-site magnetic moment. The increase in A-site
magnetic moment decreases the resultant material magnetic
moment. The substituted Mg and Zr in place of Fe3ỵions, occupy the
A-site and B-sites for their comfortablet in lattice sites. While Zr
enters A-site, it replaces the Fe3ỵions from A-site to B-site. To give
place for Fe3ỵions in B-site the Ni2ỵions are converted as Ni3ỵions
by releasing an electron. And by taking the electron from Ni ion,
Fe3ỵion is changed as Fe2ỵion [16,17]. Moreover the A-site spin
magnetic moment is always opposite to the B-site spins magnetic
moment, hence the net magnetic moment is decreased with
increasing dopant concentration. The Fe3ỵions from A-site will be
arranged anti parallel in B-site upto certain concentration, later
Fe3ỵions from A-site will be arranged on B-site with canting
posi-tion. This gives an angle between the Aesite Fe3ỵ_{ions and}
B-site-Fe3ỵions called Y-K angle[18]. The Y-K angle is calculated by using
the following equation.

nBẳ 6 ỵ xịcos

a

YK 51  xị
Fig. 7. Variation of activation energy with dopant concentration.

Fig. 8. Magnetization versus magneticfield (M-H) curves of Zr and Mg substituted Ni0.5Zn0.5ZrxMgxFe2-2xO4. at room temperature.

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The variation of Y-K angles with dopant concentration is
shown inFig. 9b. FromFig. 9b it is clear that Y-K angles increased
with increasing dopant concentration. The increase in the Y-K
angles suggests the increase of A_{eB interaction}[19]. The coercive

field is a field where the magnetization becomes zero in reverse
order. The composition variation of Coercivefield and porosity is
shown inFig. 9c. FromFig. 9c it is concluded that the increase in
the porosity increases the coercivefield. According to J. Smith and
H.P.J. Wijn the increase in the porosity of the material will affect

the reverse magnetization of material [20]. On removing the

applied magneticfield magnetic dipoles will not come to initial

orientation since the magnetic dipoles lag by field (i.e. the

magnetic dipoles suffer by residualflux). This lagging of field is
affected by the porosity of samples[21]. Hence an increase in
porosity increases the coercivefiled.

7. Magnetic permeability

Permeability is the property of a magnetic material which
measures its ability to support the formation of magneticfields
within itself. The extent of the magnetization of a material denotes
the response to an applied magneticfield. The permeability of the
material is estimated from the following equation

m

¼ L
L0

Fig. 9. Variation of net magnetic moment and saturation magnetization (a), Y-K angles (b), coercivefield and porosity (c), and permeability and grain size (d) with dopant
concentration.

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where L is the measured inductance of the torroids and Lo is
calculated as follows

Lo¼ 4:606N2log


OD
ID


t 109Henry

Fig. 9d shows the variation of the initial permeability and grain
size with the dopant concentration. FromFig. 9d it is clear that the
permeability decreases with increasing dopant concentration, since
the permeability and the grain size are related as shown in the
following equation.

K¼ cd2

here K is the permeability, c is the dimension less constant and d is
the grain size. Hence permeability is directly proportional to the
square of the grain size. The materials composed with grains
include the atomic dipoles or spin dipoles[22]. The existed atomic
dipoles or spin dipoles in grains are oriented randomly. By the
application of externalfields, the atomic dipoles or spin dipoles in
grains align with thefield direction. This is related to the induced

magneticflux in the magnetic material, which continues to

in-crease up to a certain frequency. In this case, a resonance peak will
appear where the frequency of the appliedfield equals the spin
dipoles or atomic dipoles. It means that the maximum magnetic
flux is induced in the material at the resonance frequency. Later

atomic dipoles will not follow the applied _{field. According to}

literature survey the resonance peak appearance connects to: (i)
inhomogeneous material, (ii) crystalline magnetic anisotropy, (iii)
the combination of the magnetic anisotropy and the

ferromag-netic exchange fields, (iv) the domain walls and (v) the

elec-tromagnetic body resonance. The frequency variation of

permeability in the present study is shown inFig. 10. From the

Fig. 10 it is clear that all samples exhibit the resonance peak
around the 12 MHz to 13 MHz. The resonance effects occur in all
ferrous and paramagnets. In particular, it is not peculiar to ferrites
in its simple form. Generally, it has been considered that the
resonance in ferrites accounts for a large part of the magnetic
dispersion of ferrites. The permeability arises from the rotation of
magnetic dipoles rather than from a domain wall displacement
process. The domain wall displacement does not account for
magnetic dispersion at higher frequency[23]. More complex type
resonances are considered in ferromagnetic and
antiferromag-netic materials, the sample shape anisotropy is taken into account
in this case. Sometimes there may be two or more resonance
frequencies possible in an accessible range of frequency. In this

case, domain wall effects will be considered. The moving wall sets
up a magnetic double layer in the wall, and as a result the
addi-tional energy acquires its static value. Another resonance
fre-quency appears due to body-resonance which accounts for the
high permeability and high permittivity[24]. Both are comparable
at body resonance frequency and as a result, the permeability and
permittivity of a material may give too small values. The
perme-ability and anisotropy constant are shown inFig. 11with dopant
concentration. The permeability and anisotropy constant are
related as shown in following expression.

m

i∞
M2_sD

K

From the above relation, the initial permeability is directly
proportional to the square of saturation magnetization, and
inversely proportional to anisotropy constant. The permeability
strongly depends on the homogeneity of the material. That means,
permeability depends on the grain size, intra and intergranular
porosity of material. If they are not explained well, then the

Table: 1

The structural data of Zr and Mg substituted Ni0.5Zn0.5ZrxMgxFe2-2xO4.

Dopant
concentration
Lattice

parameter(Å)
Crystallite
size (nm)
X-ray
density g/cm3

Porosity (%) Sintered
density g/cm3

Grain
size (mm)

Particle
size (nm)

0.08 8.2997 5.3857 5.5237 10.99 4.9166 2.2286 25.1005

0.16 8.2996 5.3112 5.6840 14.34 4.8685 2.1454 27.7178

0.24 8.3072 5.1105 5.7482 16.16 4.8191 2.0556 29.6136

0.32 8.3367 5.5166 5.7665 17.31 4.7682 1.9542 32.3956

0.4 8.3908 5.5663 5.733 19.12 4.6370 1.8775 37.3752

Table: 2

The DC resistivity data of Zr and Mg substituted Ni0.5Zn0.5ZrxMgxFe2-2xO4.

Dopant concentration DC resistivity(r)

U-cm

Drift mobility(h) 1036 _{Activation energy (}_D_{E) eV}

0.08 1.92775E6 1.5834 0.1765

0.16 889033 1.7539 0.1673

0.24 430824 2.9249 0.1509

0.32 243293 4.62057 0.1411

0.4 89791 6.1224 0.1172

Table: 3

The magnetic properties of Zr and Mg substituted Ni0.5Zn0.5ZrxMgxFe2-2xO4.

Dopant concentration Net Magnetic
momenthB

Saturation
magnetization emu/gm
Coercive
field (Hc)Oe
Anisotropy
constant(K)

Y-K angles (⁰) Permeability

0.08 5.8800 65.4921 11.4795 80.1051 35.3507 114

0.16 5.6074 60.5877 12.8929 105.62 42.7818 84

0.24 4.8321 51.4706 30.7264 117.13 48.4878 52

0.32 3.9987 42.3480 43.6102 134.71 54.1299 33

0.4 3.2924 34.7691 44.2298 200.68 58.5468 25

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permeability mechanism must be governed by some other

mechanism of magnetic anisotropy and magnetostriction [25].

There are three types of magnetic anisotropy (1) crystal structure,
(2) grain shape and (3) applied stress or residual stresses. The
crystal structure anisotropy is independent of grain size and shape
and it can be easily observed by measuring the magnetic curves in
different crystal directions. The interaction of the spin magnetic
moment with the crystal lattice gives easy and hard directions.
The magnetized body produces the poles or charge distribution at
the surfaces. As a result, the magnetized body itself acts as another

source of the magnetic field called a demagnetizing field. This

demagnetizingfield acts opposite to the magnetizing field, and it

happens by applying a magneticfield in the material hence the

permeability decreases. In short, decreasing the shape of

magnetizing body effects the permeability. In case of round
sha-ped magnetized body, the anisotropy constant will be higher and
for cube shape, the anisotropy constant will be low. The
perme-ability will be low for round shaped magnetize body, and for cube
shape it will be higher. FromFig. 4, magnetized bodies other than
the cube shape should be grown, so that the anisotropy constant
will be high. As a result, the permeability decreases with
increasing dopant concentration. The third one arises due to the
spin-orbit coupling that produces strain along the
crystallo-graphic axis. So, the magnetized body will change directions when
magnetized[13].

8. Conclusion

The Zr and Mg co-substituted Ni0.5Zn0.5Fe2O4 ferrites have
been prepared by sol-gel auto combustion method. The
investi-gated samples revealed the semiconducting nature, in which the
activation energydecreased with increasing dopant
concentra-tion. The XRD patterns confirm the single phase cubic spinel and
no secondary phase was identified by this analysis. The lattice
constant, cell volume as well as the porosity of samples increased
with increasing the dopant concentration. The density of material,
as a consequence, decreased by pores developed in the material.
The crystallite and particle sizes are comparable in the nano scale.
As the dopant content varies, the DC resistivity and drift mobility
varied in the opposite way. The saturation magnetization, net
magnetic moments and permeability are reduced with increasing

dopant concentration, while the coercive field and anisotropy

constant are enhanced. The Y-K angles increase with increasing
dopant concentration. Electrical and magnetic properties have
been discussed in good correlation with the structural behaviour
(seeTables 1e3).

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